1. Field of the Invention
The present invention relates to a method for forming a spigot end on double-walled pipe, as well the spigot ends formed by this method.
2. Discussion of the Background
Double-walled pipes are well-known. For an example of a production method of commercial significance, please see Petzetakis et al. (U.S. Pat. No. 3,917,500) which is hereby incorporated by reference. Despite the mechanical advantages of double-walled pipes, problems with handling exist. One handling problem arises during joining neighboring sections of pipe to produce a continuous longer pipe. Joining is commonly performed with a xe2x80x9cbell and spigotxe2x80x9d joint. One end of one of the sections of pipe that will be joined (the spigot) is endowed with a smaller outer diameter than the inner diameter of one end of the other section of pipe (the bell). This can be done during manufacture of the neighboring sections of pipe, or it can be done after cutting and other preparation unique to the location where the sections of pipes will be used. Regardless of when and how the bell and spigot are formed, joining involves the insertion of the smaller outer diameter portion of pipe (spigot) into the larger inner diameter of pipe (bell). Sealing a bell and spigot joint to prevent leakage can be done in many ways, and is highly dependent upon materials and conditions.
Sealing a double-walled pipe has proven to be relatively difficult. Because of the existence of one or more continuous channels between the inner and outer wall of a double-walled pipe in many commercially important double-walled pipes, both the inner and the outer wall of both portions must be sealed at each joint. Otherwise, the channel(s) may serve as a path for mass transport of material between the interior and the exterior of a pipe. This mass transport, otherwise known as leaking, can result in waste, damage to other infrastructure, excessive pressure drops along the length of the pipes, or health and safety concerns depending upon the material carried in the pipes.
Various methods for sealing double-walled pipes have been proposed, and the ones discussed herein are hereby incorporated by reference. Valls (U.S. Pat. No. 5,099,888) teaches the sealing of double-walled pipes by xe2x80x9cinserting an annular elastomeric plug into a slot formed at the pipe ends, whereby the plug projects axially beyond the pipe end to provide impact resistance.xe2x80x9d (col. 2, lines 12-15) This annular plug xe2x80x9cis particularly designed and configured for securement within a groove or circumferential slot formed by removal of at least one of the ribs adjacent to the end of the pipe where the annular ring is being providedxe2x80x9d (col. 2, lines 49-53).
Durrenberger et al. (U.S. Pat. No. 5,096,528) teaches the sealing of double-walled pipes when xe2x80x9ca substantial portion of the intermediate wall which lie[s] between the side walls axially inward of the outer end of the pipe is removed for a distance substantially equal to the desired length of the chamfer and circumferentially of the pipe to define a circumferentially continuous, axially outward open channel between the side walls. After the intermediate wall portions are removed, the outer end of at least one of the side walls is deflected to a position wherein it substantially engages the other side wall and closes the outwardly open channelxe2x80x9d (col.1, line 61-col.2, line 3).
It should be noted that both of the above-mentioned references require the formation of a xe2x80x9ccircumferential slotxe2x80x9d through the removal of the helical rib(s) (otherwise called an xe2x80x9cintermediate wallxe2x80x9d) located between the inner wall and the outer wall at the end of the double-walled pipe. Furthermore, both references teach the sealing of this circumferential slot rather than the sealing of the helical channel(s) defined by the helical rib(s). In both references, sealing this circumferential slot is performed by deflecting the outer wall toward the primary axis of the double-walled pipe. Since the initial diameter of the outer wall prior to deflection is larger than the final diameter of the outer wall after deflection, the outer wall must be compressed in order to achieve this deflection. In both references, this compression is ameliorated by heating the end of the double-walled pipe.
In the Durrenberger et al. (U.S. Pat. No. 5,096,528) reference, an additional adhesive layer is required on the outer face of the inner wall at the end of the double-walled pipe to affix the inner wall to the outer wall. Furthermore, the chamfer formed by the deflection of the outer wall is constrained to have the material properties of the outer wall of the double-walled pipe. In other words, since the spigot is comprised of the same material as the outer wall, the sealing and flexibility of the spigot cannot be optimized for a joint. Rather, the material properties of the joint are the same as the materials properties of the entire double-walled pipe.
In the Valls (U.S. Pat. No. 5,099,888) reference, an annular ring is inserted into the circumferential slot to both form a seal and provide some degree of control over the material properties of the joint. However, Valls teaches compression sealing of the annular ring between the inner and outer wall of the double-walled pipe, despite the fact that any residual stress in the outer wall due to deflection toward the primary axis will oppose this seal.
Accordingly, one object of this invention is to provide a novel method for forming a spigot end on a double-walled pipe that is applicable to a variety of types of double-walled pipes.
A further object of this invention is to provide a method for forming a spigot end on a double-walled pipe that does not require the formation of a circumferential slot at the end of the pipe.
A further object of this invention is to provide a method for forming a spigot end on a double-walled pipe with a porous intermediate layer, wherein the channel or channels between the inner and outer walls are not pressure sealed.
A further object of this invention is to provide a method for forming a spigot end on a double-walled pipe that itself is formed by the contiguous winds of a helix of a smaller diameter pipe or pipes, wherein the channel of the smaller diameter pipe or pipes is sealed rather than the area between the inner wall to the outer wall.
A further object of this invention is to provide a method for forming a spigot end on a double-walled pipe having a porous intermediate layer, wherein the porous intermediate layer is accessible during the spigot-forming process.
A further object of this invention is to provide a method for forming a spigot end on a double-walled pipe wherein deflection of the outer wall is not required.
A further object of this invention is to minimize the number of tools and steps, as well as the degree of difficulty, used in forming a spigot end on a double-walled pipe.
A further object of this invention is to provide a spigot end on a double-walled pipe that is formed by a method incorporating the advantages described above.
A further object of this invention is to provide a spigot end on a double-walled pipe that seals a porous intermediate layer with a minimum of sealant and that seals as small an area as possible.
A further object of this invention is to provide a spigot end on a double-walled pipe wherein the properties of the chamfer are both optimized and easily varied for a range of applications.
To these ends, the present invention provides a method whereby a spigot end is formed upon a double-walled pipe by first removing substantially all of the outer wall and the intermediate layer from one end of the double-walled pipe over a length substantially equal to the chamfer length to form an annular ledge consisting substantially of the inner wall. An annular resilient bevel is then fixed around this annular ledge to form a spigot that has the material properties of this bevel, rather than the properties of the outer wall of the double-walled pipe. Furthermore, since the outer wall of the double-walled pipe is not radially compressed to form all or a portion of the chamfer, it need not be heated to ameliorate any compressive stress arising from this process. This limits the number of steps and types of tools needed to form a spigot join, and eliminates residual stress that may prove problematic during sealing.
Sealing a channel or channels that exist between the inner and outer wall of a double-walled pipe is both simplified and improved by the present invention. Since substantially all of the outer wall and the intermediate layer are removed, any channel or channels intermediate to the inner and outer wall are easily accessible at a position substantially equal to a chamfer length distant from the spigot end. Thus, the channel(s) can be plugged, rather than sealing an entire circumferential slot. Plugging the channel(s) directly will result in a higher quality seal using a smaller amount of material.